I don't know about Promasters, but I'm on the roof of our T1N Sprinter all the time. I wash it but don't wax it any longer because I Bus Koted
it. Once a year I dismantle the Fantastic cover for cleaning, and I also refresh the Bus Kote annually with a thin overlay.
I've never heard of anyone damaging the roof of a Sprinter by being on it, but I suppose its always possible.